In a conventional method of manufacturing a semiconductor monocrystal, such as silicon monocrystal, based on the known Czochralski technique, a series of monocrystal ingot pulling operations mainly comprise the following steps: (i) a batch of semiconductor substance (e.g., silicon) is charged in the crucible, (ii) the crucible is heated by means of resistance heating, or high-frequency heating, whereby the semiconductor substance is melted, (iii) a seed crystal fixedly held by a pulling means is dipped in the surface of the melt, (iv) the pulling means is slowly turned and lifted up at a raising rate of several millimeters per minute whereby the grown monocrystal is pulled up above the surface of the melt, (v) the steps (iii) and (iv) are repeated until the melt in the crucible is nearly depleted, (vi) the crucible is cooled down so that the crucible can be charged with a new batch of semiconductor substance.
However, in the above manufacturing operation based on a batch charging system, since the furnace as well as the crucible must be cooled down to allow replenishment of the crucible each time the crucible is nearly empty, the number of monocrystal ingots raised in a series of the operation is small and therefore rate at which the ingots are produced is so small that it is difficult to improve the productive efficiency, furthermore, the risk of breakage of the expensive crucible is relatively high due to the difference in thermal expansion coefficient between the crucible and the semiconductor substance therein. Therefore, it is desirable that the semiconductor substance is fed in the melt held in the crucible between pulling operations so as to relieve the crucible from the necessity of being frequently cooled down.
In the above operation of the conventional manufacturing apparatus for raising grown semiconductor monocrystal ingot, one series of monocrystal raising steps is terminated when a limited number of monocrystal rods have been raised; during the downtime the crucible is replenished with the high-purity semiconductor polycrystal substance, the weight of which is equal to the weight of the grown and raised monocrystal substance; and thereafter the next series of the monocrystal raising steps is initiated. This operation of replenishing is conventionally called as "recharging", for the crucible is charged with the semiconductor polycrystal substance anew.
As already stated, in this procedure of recharging, it is necessary to let the whole furnace cool down before recharging the crucible with fresh semiconductor substance, so that this procedure is time-consuming and acts as a bottleneck impeding productive efficiency. Also, the increased frequency of heating and cooling of the expensive crucible renders the crucible more liable to breakage. However, if the furnace were kept uncooled during the recharging in the open air and thus if the remnant semiconductor substance in the crucible were kept molten, the molten semiconductor substances (e.g., molten silicon) and the intra-furnace carbon-made structural members as well as insulator kept at high temperatures could burn in the air. Therefore, while the furnace is being heated, it is absolutely impossible to replenish the hot crucible in ambient air in the same manner as the initial batch of the semiconductor substance is charged in the crucible before heating the furnace.
Various proposals have been made of recharging devices which enable additional of semiconductor substances to a crucible without the necessity of having a furnace cooled down. For example, the devices disclosed by Japanese Provisional Patent Publication (Kokai) No. 57-95892 (1982), Japanese Provisional Patent Publication (Kokai) No. 58-95548 (1983), and Japanese Provisional Patent Publication (Kokai) No. 58-170533 (1983) are basically related to the same system comprising a hollow metallic cylinder internally lined with quartz and having a movable door means in the bottom which is opened by means of an operation of a wire system. Lumps of semiconductor substance are dropped from the door. However, it is necessary to solidify the surface of the melt before dropping the semiconductor substance from this system, and a complicated operation is required so as to prevent several lumps of semiconductor substance from falling at once, and, what is more, a quartz-lined pipe must be provided, which makes the construction of the monocrystal ingot pulling apparatus extremely complicated. Also, in these proposed devices, since lumps of semiconductor substance are used as the additional supply, it is necessary to sort them by size, and this creates an opportunity for contamination of the high-purity semiconductor substance.
Another method is also proposed wherein a fresh semiconductor substance is supplied in the form of a rod. In this method, a rod of the semiconductor substance is suspended from a pulling means via a wire one end of which is tied at the top portion of the rod, and the rod is lowered at a constant rate to thereby cause the bottom portion of the rod to be submerged and gradually melted in the melt contained in the crucible. However, in this system the top portion of the rod at which the wire is tied cannot be dipped in the melt for if the wire melts in it the purity of the semiconductor melt is lowered. As a result, the top portion of the rod is always left unused and this creates a difficulty of obtaining a precise controlling of the weight of recharging of the semiconductor substance.
The present invention is intended to solve the above problems, and it is an object of the invention to provide a recharging device which is simple in construction as well as in operational procedure, and with which it is possible to improve the productive efficiency.